1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical device which can detect a relative inclination between an objective lens and a recording medium.
2. Description of the Related Art
In general, an optical pickup device records/reproduces information signals while it moves across a recording medium such as a disk which is mounted on a turntable and rotates. When the rotating disk is inclined due to unevenness of the weight distribution, the recorded/reproduced signals deteriorate.
In particular, in the case of an optical pickup device adopting a light source for emitting light having a relatively short wavelength and an objective lens having a greater numerical aperture, in order to increase recording density, coma aberration is generated much due to the inclination of a disk so that deterioration of recording/reproducing signals is more severe.
An apparatus for detecting and correcting a relative inclination between a disk and a corresponding lens, as shown in FIG. 1, has been suggested to prevent the deterioration of the recording/reproducing signals.
Referring to FIG. 1, a holder 10 for supporting an objective lens 3 is movably installed at a main body 20. A first tilt sensor is installed on the outer surface of a top side portion 22 of the main body 20, facing a disk 1, while a second tilt sensor is installed on the inner surface of the top side portion 22. A reflection plate 17 is installed on the upper surface of the holder 10, facing the second tilt sensor. Here, coil members (not shown) are installed around the objective lens 3 fixedly installed at the holder 10 so that the objective lens 3 can be actuated by interaction with a permanent magnet 21 installed at one side wall of the main body 20. Although it appears in the drawing that the top side portion 22 is integrated with the main body 20, a cover member (not shown) is generally provided instead of the top side portion 22.
The first tilt sensor includes a light emitting diode 11, and a pair of photodiodes 12 and 13 disposed at either side of the light emitting diode 11, aligned in the radial direction. Light emitted from the light emitting diode 11 is reflected by the surface of the disk 1 and then received by the photodiodes 12 and 13. The amounts of light detected by the photodiodes 12 and 13 are the same when the disk 1 is not inclined with respect to the main body 20. When the disk 1 is inclined, the amounts of light detected by the photodiodes 12 and 13 are different from each other.
Likewise, the second tilt sensor includes a light emitting diode 14, and a pair of photodiodes 15 and 16 disposed at either side of the light emitting diode 14, aligned in the radial direction. Light emitted from the light emitting diode 14 is reflected by the reflection plate 17 installed on the upper surface of the holder 10 and then received by the photodiodes 15 and 16. The amounts of light detected by the photodiodes 15 and 16 are the same when the holder 10 is not inclined with respect to the main body 20. When the holder 10 is inclined, the amounts of light detected by the photodiodes 15 and 16 are different from each other.
In the above first and second tilt sensors, detection signals from the photodiodes 13 and 15 and the photodiodes 12 and 16 are input to (+) and (xe2x88x92) input terminals of a differential amplifier 18 where the signals are operated.
Thus, as shown in FIG. 2A, when the disk 1 and the objective lens 3 are not inclined, since the detection signals of the photodiodes 12 and 13 are the same and the detection signals of the photodiodes 15 and 16 are the same, an output signal value of the differential amplifier 18 is zero, which means that there is no relative inclination between the disk 1 and the objective lens 10.
As shown in FIG. 2B, when the disk 1 is inclined to the main body 20 and the holder 10 is not inclined to the main body 20, the amount of light detected by the photodiode 12 increases while the amount of light detected by the photodiode 13 decreases. Here, the amounts of light detected by the photodiodes 15 and 16 are the same. In this case, the differential signal of the differential amplifier 18 is a positive value according to the difference of the detection signals of the two photodiodes 12 and 13.
To correct the inclination of the disk 1, the holder 10 is made to be inclined to the same degree and in the same direction as that of the disk 1, as shown in FIG. 2C. Then, the amounts of light by the photodiodes 12 and 13 of the first tilt sensor are different from each other and simultaneously the amounts of light by the photodiodes 15 and 16 of the second tilt sensor are different from each other. However, the differential signal of the differential amplifier 18 is zero, which signifies that the inclination of the disk 1 is corrected and there is no relative inclination between the disk 1 and the objective lens 3.
As described above, in the conventional disk inclination detecting apparatus, the inclinations of the disk 1 and the holder 10 are detected and the objective lens 3 mounted on the holder 10 is made to be inclined with respect to the main body 20 to the same degree as the inclination of the disk 1 so that a relative inclination of the disk 1 to the objective lens 3 can be corrected.
However, in the above disk inclination detecting apparatus, since two or more additional tilt sensors are needed, the number of parts are many and the manufacturing cost is high. Also, since the reflection plate 17 is installed at the holder 10, a load on the actuating mechanism increases so that the actuating performance of an optical pickup device is lowered.
To solve the above problems, it is an objective of the present invention to provide an optical pickup device which can detect a relative inclination between an objective lens and a recording medium without an additional tilt sensor.
Accordingly, to achieve the above objective, there is provided an optical pickup device which comprises a light source for generating and emitting light, optical path changing means for changing a proceeding path of incident light, an objective lens for focusing incident light on a recording medium which has a recording surface where information is recorded and a protection layer, at the side where light is input, for protecting the recording surface, a main photodetector for detecting the information signal and/or an error signal by receiving the light reflected by the recording surface of the recording medium and passing through the objective lens and the optical path changing means, and an inclination detecting unit for detecting inclination of the recording medium by receiving the light emitted from the light source and reflected by the surface of the protection layer or the recording surface of the recording medium.
It is preferred in the present invention that the inclination detecting unit comprises a holographic lens, disposed on the optical path between the light source and the optical path changing means, for diffracting and transmitting light input from the light source, and an inclination photodetector, disposed at one side of the main photodetector to receive incident light reflected by the surface of the protection layer and passing through the objective lens and the optical path changing means, and formed of a plurality of split plates, each performing photoelectric-conversion independently, the split plates being arranged in a track tangential direction and/or a radial direction of the recording medium.
Also, it is preferred in the present invention that the holographic lens comprises a first transmitting portion for transmitting light a portion of the light emitted from the light source within a near-axis area used for recording and reproducing information signals, and a second transmitting portion, formed around the first transmitting portion, for transmitting a portion of the incident light withing a far-axis area, wherein one of the first and second transmitting portions allows incident light to pass straight therethrough and the other one diffracts and transmits the incident light to +1st or xe2x88x921st order.
Also, it is preferred in the present invention that the inclination detecting unit comprises at least one inclination photodetector having a plurality of quadrant plates arranged in the track tangential direction and/or radial direction of the recording medium at at least one side of the main photodetector, to detect the light reflected by the surface of the protection layer.
Also, it is preferred in the present invention that the inclination detecting unit further comprises a holographic lens which is disposed on the optical path between the optical path changing unit and the objective lens and includes a first transmitting portion which is provided such that most light transmitting the first transmitting portion is the light reflected by the recording surface of the recording medium, and a second transmitting portion, formed around the first transmitting portion, for transmitting light reflected by the surface of the protection layer of the recording medium, wherein one of the first and second transmitting portions allows incident light emitted from the recording medium to pass straight therethrough and the other one diffracts and transmits the incident light emitted from the recording medium to +1st or xe2x88x921st order.
Also, it is preferred in the present invention that the optical pickup device further comprises a collimating lens, disposed on the optical path between the holographic lens and the objective lens, for collimating the light output from the light source and passing straight through the first transmitting portion, so that light, excluding a part thereof from the outer circumference of the incident light reflected by the recording medium, can pass the collimating lens.
Also, it is preferred in the present invention that the inclination photodetector is provided at both sides of the main photodetector along the track tangential direction and/or the radial direction.